单选题   Para. 1 When the world looks at the United States, it sees a land of exceptions: a time-tested if noisy democracy, a crusader in foreign policy, an exporter of beloved music and film.
    Para. 2 ①But there is one quirk that consistently puzzles America's fans and critics alike. ②Why, they ask, does it experience so many mass shootings?
    Para. 3 ①Perhaps, some speculate, it is because American society is unusually violent. ②Or its racial divisions have frayed the bonds of society. ③Or its citizens lack proper mental care under a health care system that draws frequent derision abroad.
    Para. 4 ①These explanations share one thing in common: Though seemingly sensible, all have been debunked by research on shootings elsewhere in the world. ②Instead, an ever-growing body of research consistently reaches the same conclusion.
    Para. 5 The only variable that can explain the high rate of mass shootings in America is its astronomical number of guns.
    Para. 6 The top-line numbers suggest a correlation that, on further investigation, grows only clearer.
    Para. 7 ①Americans make up about 4.4 percent of the global population but own 42 percent of the world's guns. ②In the past decade, 31 percent of the gunmen in mass shootings worldwide were American, according to a study by Adam Lankford, a professor at the University of Alabama.
    Para. 8 ①Worldwide, Lankford found, a country's rate of gun ownership correlated with the odds it would experience a mass shooting. ②This relationship held even when he excluded the United States.
    Para. 9 The United States also has some of the world's weakest controls over who may buy a gun and what sorts of guns may be owned.
    Para. 10 ①Switzerland has the second-highest gun ownership rate of any developed country, about half that of the United States. ②Its gun homicide rate last year was 7.7 per million people—unusually high, in keeping with the relationship between gun ownership and murders, but still a fraction of the rate in the United States.
    Para. 11 The United States is one of only three countries, along with Mexico and Guatemala, that begin with the opposite assumption: that people have an inherent right to own guns.
    Para. 12 The main reason U.S. regulation of gun ownership is so weak may be the fact that the trade-offs are simply given a different weight in the United States than they are anywhere else.
    Para. 13 ①After Britain had a mass shooting in 1987, the country instituted strict gun control laws. ②So did Australia after a 1996 incident. ③But the United States has repeatedly faced the same calculus and determined that relatively unregulated gun ownership is worth the cost to society.
    Para. 14 That choice, more than any statistic or regulation, is what most sets the United States apart.
    Para. 15 ①'In retrospect Sandy Hook marked the end of the US gun control debate,' Dan Hodges, a British journalist, wrote in a post on Twitter two years ago, referring to the attack that killed 20 young students at an elementary school in Connecticut. ②'Once America decided killing children was bearable, it was over.'